Abstract The electroreductive film formation process for trans(Cl)-Ru(bpy)(CO) 2Cl 2 in DMSO is disturbed at low polarisation by decoordination of chloride ligands as a result of strong DMSO–Ru(I) interaction. Unusually high chloride concentrations were measured at potentials positive of the actual film deposition step, and the chloride oxidation response in bulk electrolysis and hydrodynamic ring-disc experiments was used to probe the reaction mechanisms. Soluble intermediates were found, of which one was identified as a cationic dimer, which absorption peak at 450 nm coincides with that of reversibly dissociating pure dimer. Initial film deposition is assigned to reduction and coupling of soluble, possibly dissociated cationic, tetramer units. At full activation the two electron reduction and Ru(0) polymer formation was confirmed.